1. Field of the Invention
This invention relates generally to encoders and more particularly to rotary encoders having an encoder disk which employs optical disk technology.
2. Background of the Invention
FIG. 1 shows a conventional rotary encoder 10. While the manufacture of this encoder refers to it as an "optical" encoder, such terminology will not be used herein. Rather, herein the term "optical" shall solely refer to a disk/readout system employing the optical principle of non-contact readout based on diffraction theory such as compact disks. In contrast, encoders of the type shown in FIG. 1 typically employ a conventional light emitting diode/photodetector combination as described below. The encoder 10 includes a shaft 12 and an output terminal 14. This particular encoder is a Model 35 incremental rotary encoder manufactured by Dynamics Research Corporation.
In typical operation, the shaft 12 is attached to a device (not shown) of which it is desired to measure the rotational motion thereof. In response to rotation of the device attached to shaft 12, output signals are generated via the output terminal 14 indicative of the motion. As known by those skilled in the art, to track this rotary motion, a disk internal to the rotary encoder 10 is provided. A typical disk 20 is shown in FIG. 2. The disk 20 is attached to the shaft 12 of encoder 10 so that it rotates with the shaft 12. The disk 20 is typically glass engraved via conventional IC lithography techniques to include a plurality of lines 22 (the arrows indicate that the lines 22 and 24 extend entirely around the circumference of the disk 20). A light emitting diode (not shown) is arranged on one side of the disk 20 and a photodetector (also not shown) is positioned on the other side. The light emitting diode/photodetector combination is fixed and does not rotate with the disk 20. As the device attached to the shaft 12 is rotated, the disk 20 also rotates and the lines 22 successively pass between the light emitting diode and photodetector inducing output signals indicative of the lines passing therebetween. Each line is equivalent to one cycle. With regard to the encoder shown in FIG. 1, a disk of 3.5 inches in diameter is designed for use therewith. Although disks can be provided with a various number of lines, the maximum number of lines available for this size and type of rotary encoder is in the range of 9000 lines (the lines 22 and 24 in FIG. 2 not being to scale). This is also referred to as the "count" of the encoder and results in a maximum of 9000 cycles per shaft revolution.
FIG. 3 shows the typical outputs for the rotary encoder 10. Although various output configurations can be provided, channels A and B (as well as their complementary channels) provide the primary outputs of the encoder 10 and can alternatively be generated in a sine wave form. Output A is generated by the lines 22 in FIG. 2 as described above. Output B is generated by a second light emitting diode/photodetector combination sensing a second set of lines 24 shown in FIG. 2. This second set of lines 24 is identical to the first set of lines 22 except that it is 90.degree. out of phase from the first set of lines 22. Accordingly, output B is simply output A shifted by 30.degree. or a quadrature wave form of output A. By having two output wave forms of the same frequency which are 90.degree. out of phase, it is possible to determine the direction of motion of the disk 20 (FIG. 2) and, therefore, the device attached to disk 20. This is conventionally accomplished by providing the A and B signals as the "D" and "CLOCK" inputs, respectively, of a D-flip flop. As a result, if the Q output line is high, the disk is being rotated clockwise, if the Q output line is high, the disk is being rotated counterclockwise. Since the output on channels A and B provide 9000 cycles per shaft revolution, one cycle can be provided every 0.04.degree. of rotation.
As known by those skilled in the art, internal and/or external cycle interpolation can be added to increase the number of counts per revolution. With regard to the specific rotary encoder shown in FIG. 1, for example, so-called external four times circuitry can be added to provide 36,000 counts per revolution, and internal ten times circuitry and external four times circuitry can be added to provide 360,000 counts per revolution. This type of circuitry, which can conventionally be added to any type of encoder including that described herein, of course, adds considerable expense and complexity to the encoder.
In certain fields such as compact disk manufacturing, super high resolutions, in the area of hundreds of thousands of counts per shaft revolution, are required. For instance, in the compact disk field, so-called Red Book requirements mandate that dimensions such as where lead-in starts and stops and track pitch, which is in the order of 1.5-1.7 microns, be measured. Conventional rotary encoders are simply not suitable for this type of measurement absent additional circuitry. Accordingly, it would be desirable to provide a super high resolution rotary encoder not requiring additional external or internal circuitry to obtain such resolutions.